In recent years, electronic control of e.g. automobiles have been developing. Intensive efforts have been made to develop LSI chips in which digital circuits and high breakdown voltage transistors are combined into a system. DMOS (double-diffused MOS) transistor, for example, is widely known as a high breakdown voltage transistor. For example in an n-channel transistor, a p-type backgate region is formed in a surface portion of an n−-type epitaxial layer, and an n+-type source region is formed in the p-type backgate region. An n-type drain region is formed to oppose to the source region, sandwiching parts of the n− type epitaxial layer and the p-type backgate region in between. Such transistor structure having doubled structure of a source region and a surrounding backgate region is called DMOS transistor, even when no concentration gradient is formed by diffusion.
For example, an n−-type epitaxial layer is grown on a p-type silicon substrate, and a p-type backgate region is formed in the n−-type epitaxial layer. An n+-type source region is formed in the p-type backgate region, and the p-type backgate region and the n+-type source region are connected to be at the same electric potential. A portion of the p-type backgate region which is exposed at the surface adjacent to the source region constitutes a channel region, and an n-type drain region is formed in the n−-type epitaxial layer opposing to the channel region with a LOCOS field insulation film sandwiched in between. An n-type high concentration region is formed between the channel region and the LOCOS field insulation film, and a gate insulation film is formed on the channel region and the n-type high concentration region. A gate electrode is formed on the gate insulation film, extending onto the LOCOS field insulation film. Thus, a DMOS transistor is formed. The high voltage applied to the drain region is relaxed in direction parallel to the surface by resistance of the n−-type region located under the LOCOS field insulation film (for example, see, Japanese Unexamined Patent Publication (Kokai) No. 2009-239096).
Substrates manufactured by using epitaxial growth are expensive. For cost reduction, it is preferable to form a high breakdown voltage transistor without using an epitaxial substrate.
There is an on-vehicle LSI called CAN (controller area network) driver. CAN is designed for communication among electronic modules through a common bus line, and a voltage is usually supplied from a battery power supply (12 V to 24 V). Even if the LSI is disconnected from the ground due to some traveling trouble, it is required not to change the bus line potential. To meet this requirement, the DMOS transistor should withstand a voltage of reverse polarity. The capability to withstand a voltage of the polarity reverse to that of the normally applied voltage is referred to as reverse voltage durability.